This invention relates in general to fasteners and more particularly to an improved fastener apparatus and method for obtaining a high clinching force and a securely maintained broken mandrel portion in installed commercial fasteners.
A blind rivet is generally composed of two pieces. The first piece is the mandrel, which is composed of a cylindrical stem having an enlarged head at one end. The second piece is the sleeve, a generally tubular member surrounding a portion of the mandrel stem and abutting the mandrel head, with an outwardly protruding flange at the end opposite the mandrel head.
In use, such a fastener is typically placed in the pulling head of a powered fastener installation device and directed to the workpiece, which commonly consists of a plurality of members to be fastened. In other instances, the fastener is placed in the workpiece and the powered fastener installation device then applied to it. In either case, the stem of the fastener is gripped by the jaws of the fastener installation device pulling head, which is then operated by hydraulic pressure to clamp the jaws radially about the fastener stem and pull the stem rearward away from the workpiece. At the same time, a reactive force is applied to the sleeve flange, urging it forward against the workpiece. The tension on the stem pulls the stem head against the sleeve, thereby upsetting the sleeve. The sleeve may be upset by collapsing it, as shown in U.S. Pat. No. 3,230,818, issued Jan. 25, 1966 to G. Siebol, or by splittinq it, as shown in U.S Pat. No. 3,114,921, issued Dec. 24, 1963 to A. Carusi.
Once the sleeve has been upset, the sleeve flange and mandrel head clamp the members of the workpiece together. After the fastener sleeve is upset in the manner described above, and the workpiece members are clinched together, resistance to the pin movement, and thus tension in the pin, increases under the continued application of the pulling force. It is conventional in the art to provide the pin with a weakened section, termed a "breakneck" groove. The pin is designed to fracture at the breakneck groove when the tension reaches a predetermined maximum, whereupon the pulling section of the stem separates from the remainder thereof disposed within the sleeve.
A longstanding problem with these blind fasteners of the prior art is that the resilient metal of the mandrel and sleeve, which is strained during setting, tends to spring back when the forces are rapidly released by the fracture of the stem at the breakneck groove. As a result, the clinching force on the workpiece is relaxed, which results in a reduction of both tensile and shear strength of the connection. The joint is likely to become loose and completely unsatisfactory in some applications.
The prior art has long recognized this problem and has developed a number of approaches to overcome it. One approach has been to provide the sleeve with a boss protruding from the face of the flange opposite the tubular portion and provide a sharp, conically-shaped groove which is undercut in the face of the flange surrounding the boss. The prior art teaches various ways to use this boss and undercut flange. For example, in U.S. Pat. No. 3,657,957, issued Apr. 25, 1972 to G. Siebol and U.S. Pat. No. 4,137,817, issued Feb. 6, 1979 to G. Siebol, it is taught that the nosepiece of the fastener installation tool urges the boss toward the workpiece until the generally triangular boss is wedged inward into a groove in the stem and the boss has completely separated from the surrounding flange. The boss forms a separate locking ring in this method.
Another variant of the boss and undercut flange is shown in U.S. Pat. No. 3,178,989, issued Apr. 20, 1965 to G. Siebol, which shows a boss and undercut flange arrangement in which the boss is urged into the mandrel stem grooves and into a flush relation with the sleeve flange without separating from the flange
A different arrangement is shown in U.S. Pat. No. 3,192,821, issued July 6, 1965 to G. Siebol, in which the boss is undercut rather than the flange. As shown in FIGS. 10 and 11 of the '821 patent, application of the fastener nosepiece to the boss causes the boss material to be deformed into the mandrel stem grooves as a result of the moment induced by the undercut in the boss.
The problem with all of the aforementioned prior art is that each requires an undercut, either in the flange or in the boss. Such an undercut is virtually impossible to form in the conventional cold-forming process used to manufacture blind fastener sleeves. Rather, these undercuts are formed by a separate machining step. The addition of this machining step to the manufacture of the fastener renders these fasteners prohibitively expensive for general commercial use. Indeed, the application of such fasteners has been limited to use in relatively expensive aerospace applications.
In order to provide an economical but positive lock between the sleeve and mandrel, the prior art has looked to the design of the fastener installation tool rather than the fastener itself. For example, in U.S. Pat. No. 3,230,818, issued Jan. 25, 1966 to G. Siebol, a substantially cylindrical boss is shown extending from the flange. This boss is adapted to be radially compressed into the mandrel stem grooves by use of a specially designed nosepiece of the fastener installation device. Another prior art approach to nosepiece design is exemplified by the nosepieces having a protruding anvil for use with the popular Cherry T Rivet.RTM.. The protruding nosepiece anvil displaces the material of the flange itself into the grooves in the mandrel to form a positive lock.
The primary problem with the Cherry T Rivet.RTM. and the fastener of the '818 patent is that use of these fasteners requires a special fastener installation tool nosepiece. This requirement has always posed a problem in that these nosepieces are more complicated and more expensive than the standard flat nosepiece. Further, they tend to wear and break, thereby reducing the effectiveness of installation of the fastener. Because the operators of such equipment may be relatively unskilled, the worn or broken nosepiece may go unnoticed. Thus, a large number of fasteners may be improperly installed before the defective nosepiece is discovered. This can lead to very expensive repair of the workpiece with concomitant downtime.
The traditional problems with specially configured fastener installation tool nosepieces have been exacerbated by the recent application of powered fastener installation devices to automated assembly. These robot-mounted devices require a sturdy and simple design of the nosepiece so as to reduce the potential for error and downtime. The high installation volume and inability of the automated power fastener installation device to detect defects in the nosepiece render this problem critical. Thus, there exists a need for a fastener that is economical to produce, does not require a special configuration of the nosepiece of the fastener installation device, but yet achieves a high clinching force in the fastened members by preventing spring back of the fastener components.